In-situ electrochemical SERS observation was conducted at Ru/Pt alloyed overlayers formed on Au substrates with and without surface roughness. When the overlayer was formed on an electrochemically roughened Au substrate having SERS activity, spectral features of molecular adsorbates were significantly influenced by structural and compositional inhomogeneity of the surface, resulting in low spectral reproducibility under electrochemical conditions. On the other hand, when the overlayer was formed on an atomically defined Au(111), highly reproducible SERS spectra were obtained with the assistance of SERS-active sphere-plane type nano-gap structures. Application of electrochemical potentials clearly induced spectral variations such as electrochemical Stark shift of the internal molecular vibrations or appearance of molecule-to-metal charge transfer-induced Raman bands. The use of well-defined interfaces in SERS observation enabled us to obtain information on electron-vibration coupling at metal-molecule interfaces as well as vibrational information on molecular adsorption geometry. (C) 2016 Elsevier B.V. All rights reserved.